Summary Aim. Thegrowing body of evidence suggests that magnesium levels can serve as a marker of major depressive disorder (MDD), but findings from clinical trials remain inconclusive. The aim of the presented study was to determine the magnesium concentration in serum of patients with MDD (in the active stage of the disease or in remission) and to analyze the role of magnesium levels as apotential marker of the disease. Methods. Sixty-nine patients with current depressive episode, 45 patients in remission and 50 healthy volunteers were enrolled into the case-control study. The magnesium concentration was measured by flame atomic absorption spectrometry (FAAS). Results. The mean serum magnesium concentration of patients in the depressed phase was significantly higher, compared to the control group. Moreover, magnesium levels of patients in the remission were not significantly different from the concentrations recorded in the healthy volunteers. There was also a positive correlation between the magnesium levels and the severity of depression measured by the Hamilton Rating Scale for Depression (HDRS) and the Montgomery-Asberg Depression Rating Scale (MADRS). Conclusions. The obtained results may suggest a role of magnesium as a state marker reflecting the pathophysiological changes underlying MDD and accompanying severe depressive episodes.